This application is based upon and claims the benefit of Japanese Patent Applications No. 11-218014 filed on Jul. 30, 1999, No. 11-340601 filed on Nov. 30, 1999, No. 2000-161822 filed on May 31, 2000, and No. 2000-188278 filed on Jun. 22, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method for interconnecting printed circuit boards and an interconnection structure.
2. Description of the Related Art
JP-A-9-8453 discloses an interconnection structure for printed circuit boards, using an anisotropic conductive resin material. Referring to FIG. 18, this structure has an anisotropic conductive thermoplastic resin 52 being placed between a conductive pattern surface of a first printed circuit board 50 and a conductive pattern surface of a second printed circuit board 51, which is fused by applying pressure and ultrasonic waves. In this manner, a distance between lands 50a and 50b is reduced to establish an electric interconnection therebetween.
xe2x80x9cIntroductory High density Flexible Board (in Japanese)xe2x80x9d (by Kenji Numakura, published by Nikkan Kogyo Co., Ltd.) discloses at page 100 a method for interconnecting a hard printed board and a flexible board. In accordance with this method of interconnection, as shown in FIG. 19, a land 60a of a conductive pattern on the hard printed board 60 is interconnected with a land 61a of a conductive pattern on the flexible board 61 by using a solder 62. Furthermore, the flexible board 61 is adhered to the hard printed board 60 by using an adhesive 63.
However, in the interconnection structure as shown in FIG. 18, an anisotropic conductive resin film must be mounted on the surface of the printed circuit boards 50 and 51 by printing or the like. This requires an additional process step to obtain the interconnection structure, and leads to an increase in the production cost.
Furthermore, the printed circuit boards 50 and 51 are interconnected with their surfaces being faced each other while incorporating the anisotropic conductive resin material 52 therebetween. Accordingly, the resulting product suffers poor reliability because voids tend to be generated in the connecting interface between the resin material 52 and each of the printed circuit boards 50 and 51.
In the interconnection structure shown in FIG. 19, it is necessary to cover the edge portion of the flexible board 61 with a protective insulating film after interconnecting the flexible board 61 in order to assure the insulation reliability of the interconnected portion. Thus, in this case again, the production step is increased.
The present invention has been accomplished in view of the aforementioned problems. Accordingly, an object of the present invention is to provide a method for interconnecting printed circuit boards and an interconnection structure, which is improved in the reliability of the interconnection and capable of reducing the production cost.
According to the present invention, a land of a first printed circuit board is overlapped with a land of a second printed circuit board to form an interconnection portion. The first printed circuit board has an insulating substrate made of a thermoplastic resin. Then, the interconnection portion is heated at a temperature approximately higher than a glass transition temperature of the thermoplastic resin while applying a pressure to the interconnection portion. Accordingly, the land of the first printed circuit board is electrically interconnected with the land of the second printed circuit board. Simultaneously, the interconnection portion is sealed with a part of the thermoplastic resin.
Thus, the interconnection portion is sealed with the resin constituting the substrate of the first printed circuit board without using an anisotropic conductive film or the like, resulting in reduced production cost. Since the thermoplastic resin is softened and deformed to seal the interconnection portion while expelling air, voids are less likely formed at the interconnection portion. As a result, the interconnection structure has high reliability.